1. Field of the Invention
The invention relates to a method and apparatus for identifying and analyzing vapor elements, and more particularly to a surface acoustic wave gas chromatographic (SAW/GC) with a preconcentrator collector and a method of using same.
2. Related Art
In the analysis of chemical vapors there is frequently a need to detect extremely small trace amounts of a specific vapor amongst a background of different chemical species. Examples are the detection of contraband aboard the cargo of vessels being inspected by the United States Coast Guard and the detection of leaking explosive or dangerous chemicals at depositories thereof. Automated or portable testing apparatus for this purpose has generally not been available. Further, existing detectors are only able to detect chemical species at concentrations well above their ambient vapor concentrations and thus lack sufficient sensitivity.
Chemical sensors have been developed that physically change upon exposure and contain absorbing polymers selected for their affinity to absorb a group of related chemical species. One type, surface acoustic wave (SAW) delay line sensors, are the most developed and readily available. For example, one commercial supplier is Microsensor System, Inc., Fairfax, Va.
A method and apparatus for using an SAW device to detect a vapor is disclosed by H. Wohltjen in U.S. Pat. No. 4,312,228, issued Jan. 26, 1982. As described therein, the SAW device comprises a piezoelectric element having a surface coated with a polymer material selected to absorb and react with the chemical to be detected. Interaction of the chemical with the material coating of the sensing element alters one or more properties of a surface acoustic wave, and the electrodes on the piezoelectric element detect the altered wave, producing an electrical signal.
Another apparatus and method for detection and identification of chemical vapors is disclosed in U.S. Pat. No. 4,895,017. As described in a plurality of surface acoustic wave (SAW) devices, each coated with a selected polymer material, are exposed to the vapor to be analyzed. In this invention a predicted time constant (or rate) of diffusion into the polymer coating is used to identify the different chemical species. To quantitatively identify specific chemical species present in vapors an array of SAW sensors with different polymer coatings may be exposed and a pattern recognition technique utilized to identify specific species. This is described in a paper entitled "Correlation of Surface Acoustic Wave Device Coating Responses With Solubility Properties and Chemical Structure" by D. S. Ballentine, Jr., S. L. Rose, J. W. Grate, and H. Wohltjen, published in Analytical Chemistry, Vol. 58, p. 3058, December 1986.
A further patent using multiple polymer coated dispersive delay lines is disclosed by J. Haworth in U.S. Pat. No. 5,012,668, issued May 7, 1991. The use of specific absorbant polymers to sensitize the surface of a piezoelectric crystal and induce a phase or amplitude variation in a traveling acoustic wave is common to all of the prior art and this approach severely limits the performance of these vapor detectors. Multiple polymer films dilute the vapor samples and thereby limit the amount of vapor that can be detected by each film. Also, practically any type of film applied to the surface of a piezoelectric crystal introduces noise which reduces sensitivity further.
In view of such problems, the present inventors have proposed an apparatus for performing high speed detection and identification of vapor species based upon the differing vapor pressures of individual vapor species within a confined and focused high temperature gas stream. The apparatus includes a temperature programmed vapor preconcentrator for trapping condensable vapor species, a multi-port valve, a temperature programmed chromatographic capillary column, an acoustic wave interferometer for detecting adsorption and desorption of vapor species, a thermoelectric heating and cooling element for controlling the temperature of the acoustic interferometer sensor, and an electronic system controller which is described by the present inventors, i.e., Staples et al., in U.S. Pat. No. 5,289,715, which is hereby incorporated by reference. This apparatus is capable of detecting trace elements with high specificity and sensitivity. The detection can be done near real time.